Can "Freethought" be an adjective

I created this group called "Freethought Families of Chicago" and someone mentioned that the name is not grammatically correct. It should read, they say, as "Freethinking Families of Chicago." When I created the title, I had not really thought of it, the name just seemed to roll off the tongue well.

In my defense, I would say that "freethought" represents a philosophical worldview where as "freethinking" represents activity within that worldview. So to call it "Freethought Families of Chicago" would be to say that it is a group of families in Chicago who fall under the philosophy of freethought. "Freethinking Families of Chicago" would be to say that it is a group of families in Chicago who think freely.

What does everyone else think? Can I continue to use the title without feeling like a grammatical ignoramus?